The present invention relates to a back projection type television receiver wherein video images are projected on a permeable screen, and to its application in a large screen video display apparatus. More specifically, the present invention relates to a structure for holding such a screen.
An example of a prior art holding structure for a permeable screen used in a back projection type television receiver is shown in FIG. 8 and FIG. 9. FIG. 8 shows schematically a cross-sectional view of a prior art structure for holding a permeable screen. In general, a projection type television receiver is used to build a large screen video display apparatus with its large screen measuring, for example, 700 to 900 mm in width and about 500 to 700 mm in height.
In FIG. 8, the permeable screen 1 is formed of a Fresnel lens sheet 1a laminated with a lenticular lens sheet 1b. The screen 1 is held by a recessed slot 2e formed on the inner side of the periphery of a framework 2 with the peripheral end surfaces of screen 1 fixed in said slot 2e by insertion. A plastic material like acrylic resin is generally used as the material for the Fresnel lens sheet la and lenticular lens sheet 1b.
Because a large screen is held by the framework 2, it is desirable for the material used in the framework 2 to have strong mechanical strength, and a metallic material is usually used to build the framework 2. Plastic materials and metallic materials as well expand or shrink according to temperature changes. Particularly, the thermal expansion coefficient of plastic materials is much larger than that of metallic materials. Therefore, when a plastic material having such a relatively large thermal expansion coefficient is used as the screen material, the outline dimensions of the screen are changed greatly by expansion or contraction according to the changes in ambient temperature or humidity. For example, in case of a screen measuring 900 mm in width, its width changes about 3 mm in total over the temperature changes of -10.degree. to +40.degree. C. In contrast, the dimensional changes of a framework made of a metallic material due to thermal expansion or contraction total about 1 mm over the temperature changes of -10.degree. to +40.degree. C. Accordingly, on account of a big difference existing between the framework 2 and screen 1 in expansion or contraction, it has not been possible to realize a structure wherein the screen 1 and framework 2 are coupled together with a projection type television receiver incorporating a large picture screen. As a result, the screen 1 has been liable to fall off the framework 2, thereby causing a problem.
As a measure to prevent the screen from falling off due to expansion or contraction, a structure wherein the dimension of the holding part of the framework 2 is made much larger than that of the screen 1 has been proposed. More specifically, the screen 1 is better retained by increasing the depth of the slot 2e formed in the framework 2. In this case, the width of the framework 2 looking from the front, e.g. the edge width D may be about 5 mm in length. This depth is accounted for by the depth of the slot and the thickness of the material plate of the framework.
FIG. 9 shows a perspective view of a prior art large screen video display apparatus wherein a plurality of the foregoing permeable screen type television receivers are arranged in a matrix formation composed of multiple lines and rows. In FIG. 9, item 1 is the screen and item 2 is the framework. The framework 2 holding the screen 1 is coupled with a cabinet, wherein circuit components and optical components are contained, so as to cover the opening thereof completing a permeable screen type television receiver. A large screen video display apparatus for displaying one video image is completed by placing three of the foregoing television receivers in the horizontal direction next to one another and stacking the receivers in three stages with one over the other.
With the prior art large screen video display apparatus as shown in FIG. 9, the dimension of the total edge width including the neighboring edges is as much as about 10 mm. The edge width portions exist in the displayed video image as rather thick grid-like lines where no video images are carried. As a result, video images are missing from the picture frames. This creates a problem for viewers in that the displayed video images are hard to see.